By Jonathan Amos
Science reporter, BBC News
A satellite that can measure tiny variations in the Earth's gravity field will be one of Europe's most challenging space missions to date.
Goce, due for launch next year, looks like a spyplane from a movie.
Its arrow shape, fins, and electric engine help keep the satellite stable as it flies through the wisps of air still present at an altitude of 260km.
Goce data will have many uses, probing hazardous volcanic regions and bringing new insight into ocean behaviour.
The latter, in particular, is a major driver for the mission.
By combining the gravity data with information about sea-surface height gathered by other spacecraft, scientists will be able to track the direction and speed of ocean currents.
"If we want to improve our climate models then we need to improve our knowledge of how the oceans move, and Goce will help us do that," mission scientist Dr Mark Drinkwater, from the European Space Agency (Esa), told BBC News.
Rock and no roll
Most people are taught at school that the acceleration due to gravity at the Earth's surface is 9.8m per second squared - but, in truth, this figure varies around the planet depending on the nature of the material underfoot.
The planet is far from a smooth sphere; the radius of the globe at the equator is about 20km longer than at the poles. This ellipsoid is then marked by tall mountain ranges and cut by deep ocean trenches.
The Earth's interior layers are also not composed of perfect shells of homogenous rock - some regions are thicker or denser.
Such factors will cause the gravitational force at the surface to deviate from place to place by very small but significant amounts.
The Gravity Field and Steady-State Ocean Circulation Explorer (Goce) will map these differences. This information will then be used to fashion what is, in essence, an idealised globe. Scientists call it the geoid.
It is a critical reference. The geoid defines the horizontal, tracing a surface on which, at any point, the pull of gravity would be perpendicular to it. Put a ball on this hypothetical surface and it will not roll - even though it appears to have slopes.
GRAVITY - A MOVING TARGET
The 'standard' acceleration due to gravity at the Earth's surface is 9.8m per second squared
In reality the figure varies from 9.78 (minimum) at the equator to 9.83 (maximum) at the poles
The geoid is of paramount interest to oceanographers who study the causes of the "hills" and "valleys" on the sea surface.
If local gravity differences are not creating these features, then other factors such as currents, winds and tides must be responsible.
With the help of the Goce geoid, scientists will be able to tease out these details with a precision and at a resolution not obtainable with current satellite technology.
1. Goce senses tiny variations in the pull of gravity over Earth
2. The data is used to construct an idealised surface, or geoid
3. It traces gravity of equal 'potential'; balls won't roll on its 'slopes'
4. It is the shape the oceans would take without winds and currents
5. So, comparing sea level and geoid data reveals ocean behaviour
6. Gravity changes can betray magma movements under volcanoes
7. A precise geoid underpins a universal height system for the world
8. Gravity data can also reveal how much mass is lost by ice sheets
"At the moment we can see structures down to the size of 150-200km," explained Dr Jakob Flury, formerly of the Technical University Munich, Germany, and now with the University of Texas at Austin, US.
"That's nice but the oceanographers want more detail. Goce will have spatial resolution which is finer, down to 80-100km. There are 'fronts' and currents in the oceans that are at width scales of 100km."
This will help scientists to characterise boundary currents, such as the Gulf Stream, which flow along the edges of deeply sloping continental shelves.
"The problem today is that we're losing detail as a consequence of the fact that our geoids have a very poor resolution," said Dr Drinkwater.
"We're smearing out the interpretation of the currents, and, as a result, we vastly underestimate the amount of water, heat and salt that's being transported around the ocean."
To make its gravity map, Goce will use a gradiometer. This unique instrument consists of three pairs of accelerometers that will sense the tiny variations in the tug of gravity over different parts of the Earth.
The instrument's performance is phenomenal: it will register accelerations that are less than one millionth of a millionth of the g-force we experience when standing on the Earth.
But to make the most of this sensitivity, Goce has to fly so low it will flirt with the top of the atmosphere; and that has proved to be a headache for the engineers because any buffeting on the spacecraft from air molecules will introduce noise into the data.
The "test masses" that make up the accelerometers must be kept in perfect free fall all the time to produce reliable readings. So, Goce uses an innovative drag-free propulsion system that throttles a special engine up and down to make compensations - to, in effect, fly the spacecraft around the test masses.
Qinetiq's engine assembly about to go into a vacuum test chamber
But again, the thrust levels required are tiny - a continuously variable force of anywhere between one and 20 milliNewtons during the science phase of the mission.
"This is cruise control for a spacecraft, but at an unbelievable level of precision," explained Neil Wallace from the UK technology company QinetiQ, which has built the engine.
"If you imagine you are driving your car down the motorway at 100mph (160km/h) and a mosquito hits the windscreen - the amount of deceleration your car experiences, that's what our engine has to compensate for."
The long and short
Goce is one of a growing number of spacecraft to employ an electric engine.
It draws power from solar panels stuck on one side of the satellite and uses this to charge xenon atoms which are then hurled out of nozzles on the rear of the tube-like body.
Goce will carry 40kg of xenon and this is the life-limiting factor on the mission.
Once all the xenon has been used up, the spacecraft will no longer be able to maintain fine control, and it will be allowed to fall back to Earth and burn up in the atmosphere.
Esa expects the mission to last 20 months - perhaps longer. Even this short period, though, should be sufficient to gain the high-quality data needed to produce the most detailed Earth geoid ever constructed.
And it will be a powerful complement to the work already being undertaken by the US-German Grace mission.
Grace (Gravity Recovery and Climate Experiment) uses a pair of satellites to monitor how the gravity field changes over time - the evolving geoid.
It flies much higher - at about 500km - and as a consequence cannot match the resolution promised by Goce; but then the Grace twins are doing a different job.
"Grace is taking a movie and Goce is taking a high-resolution still", is the analogy used by Dr Michael Watkins, the Grace project scientist at the US space agency's Jet Propulsion Laboratory.
"The two missions use different ways to measure the gravity field, so they have different errors that are great to cross-calibrate," he told BBC News.
And increasingly science is about pulling together a broad range of datasets; the full picture only emerges when different perspectives and disciplines interweave.
To get a proper handle on sea level rise, for example, demands that ocean height measurements from satellites be combined with gravity and tide gauge readings and even GPS.
"It is the combinations of data which are the future," said Dr Drinkwater.
GRAVITY FIELD AND STEADY-STATE OCEAN CIRCULATION EXPLORER
1. The 1,100kg Goce is built from rigid materials and carries fixed solar wings. The gravity data must be clear of spacecraft 'noise'
2. Solar cells produce 1,300W and cover the Sun-facing side of Goce; the near side (as shown) radiates heat to keep it cool
3. The 5m-by-1m frame incorporates fins to stabilise the spacecraft as it flies through the residual air in the thermosphere
4. Goce's accelerometers measure accelerations that are as small as 1 part in 10,000,000,000,000 of the gravity experienced on Earth
5. The UK-built engine ejects xenon ions at velocities exceeding 40,000m/s; Goce's mission will end when the 40kg fuel tank empties
6. S Band antenna: Data downloads to the Kiruna (Sweden) ground station. Processing, archiving is done at Esa's centre in Frascati, Italy
7. GPS antennas: Precise postioning of Goce is required, but GPS data in itself can also provide some gravity field information